"Why did the mushroom go to the…

A mushroom is the spore-bearing fruiting body of a fungus. The following joke was cited in print in 1990:

Q. Why was the mushroom invited to the party?
A. Because he was a fun-guy.

The joke is often made into a bar joke, The following is from 1991:

Question: What do you call a mushroom that buys all the drinks?
Answer: A fun guy to be with.

“If you are what you eat, how many mushrooms do I have to eat to be a fun guy?” is a related joke.

Wikipedia: Mushroom
A mushroom (or toadstool) is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. The standard for the name “mushroom” is the cultivated white button mushroom, Agaricus bisporus; hence the word “mushroom” is most often applied to those fungi (Basidiomycota, Agaricomycetes) that have a stem (stipe), a cap (pileus), and gills (lamellae, sing. lamella) or pores on the underside of the cap. These pores or gills produce microscopic spores that help the fungus spread across the ground or its occupant surface.

“Mushroom” describes a variety of gilled fungi, with or without stems, and the term is used even more generally, to describe both the fleshy fruiting bodies of some Ascomycota and the woody or leathery fruiting bodies of some Basidiomycota, depending upon the context of the word.

22 April 1990, Cedar Rapids (IA) Gazette, “Send Me a Riddle!,” comics sec. (unpaginated):
Q. WHY WAS THE MUSHROOM INVITED TO THE PARTY?
A. BECAUSE HE WAS A FUN-GUY.
JAKE AUSTEN, NASHVILLE, TN

Google Groups: rec.humor
Mushroom joke
Andy Clews
1/24/91
Question: What do you call a mushroom that buys all the drinks?
Answer: A fun guy to be with.

Fun guy = Fungi, yes? Ohhh, please yerselves….

Google Groups: rec.humor
mushroom joke
.(JavaScript must be enabled to view this email address)
9/5/93
A mushroom walks into a bar.
He says to the bartender: “A round of drinks for everybody!”
The bartender says: “Hey! You’re a fun guy!”

Google Groups: uk.local.geordie
Local spot for ‘magic mushrooms’
.(JavaScript must be enabled to view this email address)
10/9/98
(...)
2. Old joke: Why was the mushroom so popular? But he was a fun guy. Boom boom.

Google Books
A Guy Goes Into a Bar ...
By Al Tapper and Peter Press
Kansas City, MO: Andrews McMeel Publishing
2000
Pg. 68:
A mushroom goes into a bar and sits down at the counter. He’s about to order a drink when the bartender takes one look at him and says: “Hey! Get out of here! ! We don’t serve mushrooms in this place!” Stunned, the mushroom exclaims, “Awh, c’mon, I’m a fun-gi!”

Google Books
The Everything Kids’ Joke Book:
Side-Splitting, Rib-Tickling Fun
By Michael Dahl
Avon, MA: Adams Media Corp.
2001
Pg. ?:
Why do they call you Mushroom?
Because I’m a fun guyI

Google Groups: alt.humor.puns
mushroom
Jon Arnold
9/24/01
A mushroom walks into a bar, sits down and orders a drink. The bartender says “We don’t serve mushrooms here.” The mushroom says, “Why not? I’m a fun guy!”

Google Books
The Mammoth Book of Really Silly Jokes
Edited by Geoff Tibballs
London: Constable & Robinson Ltd.
2011
Pg. 119:
Why did the mushroom go to the party?
Because he was a fun guy.

The Huffington Post
10 Of The Corniest Food Jokes On The Planet
By Alison Spiegel
Posted: 07/02/2014 9:43 am EDT Updated: 07/02/2014 9:59 am EDT
(...)
Mushroom walks in a bar, bartender says “Hey you can’t drink here.”
Mushroom says “Why not, I’m a Fun-gi!”

Twitter
PUNS
‏@omgthatspunny
Why did the mushroom go to the party?
‘Cos he’s a fungi!
Why didn’t he get in?
There wasn’t mushroom!
1:30 PM - 20 Jan 2016

“Why did the mushroom go to the party?”/“Because he was a fun-gi.”